Acrylonitrile (ACRN) is an important monomer for the synthesis of various polymers including acrylic fibers, synthetic rubbers, nylons, and is the starting material for acrylic acids and acrylaminde. Processes to prepare acrylonitrile are well known, and include the so-called “Sohio Process” in which propylene/propane react with ammonia and oxygen (air) over a catalyst at elevated temperatures (“ammoxidation”). Hydrogen cyanide (HCN) and acetonitrile (CH3CN) are produced as by-products.
HCN is a valuable by-product due to it vast uses as a starting material or as an intermediate. HCN is used, for example, as a starting material for the synthesis of various polymers, including polyamides, and chemicals. HCN is the starting material for metal cyanides including sodium cyanide and potassium cyanide, two compounds important in metallurgy for recovery of gold and the hardening of steel.
To increase HCN yield in the Sohio process, a technique commonly referred to as “methanol injection” may be employed. Methanol injection involves adding methanol gas to the acrylonitrile reactor or feed to the reactor to increase HCN production. A conventional Sohio process produces a weight ratio of acrylonitrile to HCN of about 9 to 1, whereas using methanol injection, this ratio can be decreased to 8 to 1. In a typical plant, use of methanol injection can result in an increase of about 10 million pounds of HCN per year with coproduction of about 360 to 400 million pounds of acrylonitrile per year.
Methanol injection has several disadvantages. Due to the burden placed on the system, overall yield of acrylonitrile can be reduced by as much as 5%. Methanol reduces propylene content of the reactor, resulting in less acrylonitrile being produced. High heat released at the catalyst surface as methanol reacts leads to catalyst deactivation resulting in more frequent catalyst replacement. Methanol can also react with ammoxidation intermediates to form reactive intermediates that can lead to polymer formation and fouling in downstream equipment.
Methanol also reacts with oxygen in the system, consuming this reagent, and forming undesired by-products, such as carbon oxides.
Less obvious disadvantages of methanol injection process are increased cost for equipment and energy due to the need to convert methane to methanol. Methanol is typically produced by reaction of methane with steam under high temperatures and pressures over a copper catalyst yielding carbon monoxide and hydrogen, commonly referred to as “synthesis gas” or “syn-gas.” The syn-gas then undergoes an additional high temperature reaction to yield methanol. It is desirable to avoid the inefficiencies of an intermediate step to convert methane to methanol while increasing production of HCN in an ACRN reaction system.
Other alcohols and ketones have been added to increase production of HCN in an acrylonitrile process. While such processes increase the HCN to ACRN ratio, the total pounds of acrylonitrile is reduced, and adding additional alcohols and ketones to the reactor, further accelerates catalyst deactivation.
HCN is a highly toxic and flammable gas. At high concentrations, risk increases for exothermic runaway reaction through polymerization and decomposition, which is a potentially explosive situation. Therefore, it is critical in any process which uses and/or produces HCN that safety must be of highest priority. Thus, when increasing concentration of HCN of a process, extreme caution is needed to ensure safe operation of the process.
High concentrations of HCN in acrylonitrile systems are relatively unstable, and solid polymeric HCN can form in the heads column, reducing column pressure. The heads column is the distillation column in which HCN and ACRN are separated. The pressure drop raises the column temperature further favoring HCN polymerization. Solid polymerization products plug equipment, such as relief systems, valves, instruments, and piping, which in turn, increase risks associated with HCN.
Downtime associated with cleaning of the solids and other downstream process equipped is increased and results in substantial costs and loss production of ACRN and HCN. In U.S. Pat. Nos. 6,296,739 and 6,793,776, Godbole discloses methods to reduce the risk of HCN polymerization based on reducing the amount of aqueous layer in the heads column. Godbole's methods include increasing the reflux ratio of HCN to ACRN by adding recycled or pure HCN to the heads column to reduce the likelihood of polymer formation, among others. Common practice is to reduce column pressure thus lowering the column temperature.
There remains a need for co-production of acrylonitrile and HCN, wherein the weight ratio of ACRN and HCN is less than that provided in a conventional Sohio process. It is further desired to be able to vary this ratio. It is still further desired to avoid any negative effects on the acrylonitrile process, such as catalyst deactivation, and on downstream recovery and purification operations. It is further desired to have efficient conversion of methane to HCN, or at least avoid equipment and energy cost of producing methanol. It is further desired to maintain efficiency of oxygen consumption and to minimize formation of undesired by-products. It is still further desired to use existing acrylonitrile recovery and purification equipment. It is further critical that any increase in HCN concentration be performed in a manner that does not compromise safety. The present invention meets these needs.